Various methods are known for determining the concentration of specific constituents in a patient's blood. Methods for measuring the concentration of blood constituents which require a blood sample to be taken are known in the prior art. Measurement methods are however also known, in which the concentration of blood constituents is measured while the blood is flowing through the hose line. These methods are used especially when the blood is flowing in the hose line of an extracorporeal blood circuit in an extracorporeal blood treatment.
International Patent Publication No. WO 2008/000433 describes a device for determining the concentration of specific blood constituents in a blood-filled, essentially transparent hose line of an extracorporeal blood circuit. The known device makes it possible in particular to determine the haemoglobin concentration and the fraction of red blood corpuscles (erythrocytes) in the total volume of the blood. During the measurement, the hose line is clamped between two parallel, plane contact faces, so that the hose is deformed at the sides lying opposite one another. With a light emitter, light of a specific wavelength is coupled through the transparent hose line into the blood, whilst the scattered or transmitted light is measured with a light detector. The haematocrit is then determined from the ratio of the intensity of the light entering into the blood and emerging from the blood.
European Patent No. EP 1 579 196 B1 describes a device for determining blood constituents, which comprises a clamping unit for clamping the hose line and a measurement unit. The clamping unit is constituted such that the clamped hose line has a square cross-section. The measurement unit comprises a plurality of light emitters and light detectors which are disposed around the periphery of the hose line. The light emitters and light detectors are disposed in such a way that the light emitters lie in a different plane from the light detectors, so that light emitters and light detectors do not lie opposite one another. For the measurement of the blood parameters, the hose line is deformed in the clamping unit. It must be ensured that the hose line does not get jammed in the clamping unit.